Air registers per se, are well known by those skilled in the art and air registers designed to regulate more than one source of combustion air are also well known. Thus, the prior art is replete with disclosures of air registers referring to so-called primary, secondary and even tertiary combustion air and various means to deliver this air to a boiler. Although each inventor of air registers defines and refers to combustion air in accordance with his own background and concepts, combustion air usually falls into two broad categories. Primary air is generally understood to refer to air which is used as a carrier medium to transport the fuel to the furnace. Powdered coal is a good example of a fuel which is transported in a stream of air through a separate fuel pipe. Since transport air only supplies about 18% of the air needed for full combustion, an additional supply of air must be brought into the boiler from another source and by another means. This "make up" air is sometimes called "secondary air", and is supplied in sufficient volume to provide the additional 82% of required air. However, if a boiler is oil or gas fired, usually little or no supplemental means are required to transport these fuels since they are generally self-transporting. Thus, substantially 100% of the combustion air must be supplied from some source. Whether air from this source is labeled primary or secondary is a matter of semantics. It is obvious that the source of air supplied through a given register in a powdered coal burning boiler might be referred to as "secondary". On the other hand, a source of air supplied through the same register for use in the same boiler while being operated on oil might be referred to as primary air. In the alternative, the source of air through the air register may be called secondary irrespective of whether the boiler is being coal fired, oil fired or gas fired. One fact is indisputable, the function of the air remains the same irrespective of its label.
For many years a so-called "daisy chain" type damper air register was widely used in the energy generating industry. Examples of this type of register are shown in U.S. Pat. Nos. 2,320,576 and 2,838,103. The dampers for this type of register were positioned immediately adjacent to the throat of the boiler and the complex damper operating mechanism known as a daisy chain has a history of high incidence of failure due to overheating. After a short period of operation the daisy chain linkage would freeze, rendering the dampers inoperable. Thus, operators were afraid to close the dampers during the firing of a boiler because of the high probability that they would freeze in the closed position, thereby rendering the air register inoperable. As a consequence, the daisy chain dampers were usually left wide open at all times, which rendered the register useless as a means of obtaining and controlling efficient fuel combustion.